Fatigue life has always been a significant concern with respect to rivets in high performance applications, such as on an aircraft. It is known that fatigue life can be increased for aircraft applications, often significantly, by an interference fit between the rivet and the particular aircraft part in which the rivet is installed. An interference fit is obtained with a rivet which is initially somewhat smaller than the diameter of the opening in the aircraft part hole and requires significant opposing pressure during rivet installation in the axial dimension of the rivet, i.e. longitudinally of the rivet, and with specially shaped rivet die at each end of the rivet. The dies are positioned on the ends of opposing rivet rams which produce the rivet upset. The interference fit results in the diameter of the opening in the aircraft part being slightly increased, with the aircraft part material in the vicinity of the hole being maintained in a state of compression which in turn results in an enhanced fatigue life.
One undesirable consequence of an interference fit, particularly when automatic riveting machines are used, is the phenomenon known as rivet shanking, also referred to as "head-gapping". Rivet shanking occurs when the rivet forming die on the end of the ram is moved away from the head of a rivet after the rivet has been installed. The head of the rivet will, under certain conditions, move or rebound slightly away from the surface of the part once the forming die is withdrawn, leaving a space (a gap) between the underside of the rivet head and the part surface. The gap distance, i.e. the distance between the underside of the rivet head and the aircraft part, will vary from rivet to rivet, but can exceed the maximum gap allowable by aircraft manufacturers, which is typically between 0.0015 and 0.0030 inches. This phenomenon is often referred to as rivet shanking because the shank of the rivet, which is the cylindrical portion thereof, is in the opening in the part and a rivet is rejected if the inspector can slide a feeler gauge of a particular thickness, such as 0.002, underneath the head of the rivet to contact the shank.
Typically, the tendency toward shanking is increased when the ratio of rivet length to rivet diameter is relatively large. Further, the newer aluminum alloys used in aircraft, while possessing higher strength, are also more elastic, resulting in an increase in shanking.
There have been many different attempts, several quite sophisticated and complex, to solve the rivet shanking problem. However, to date, no method has been particularly successful.